OF NORTHtJMBEBLAND AND DURHAM. 7 



Survey of Great Britain," have demonstrated the use of local 

 Faunas like the present, in furnishing materials for working out 

 some of the most interesting problems that Natural History af- 

 fords ; but for the true solution of problems of this kind, it is not 

 only necessary that local Catalogues should be as complete as 

 possible in the productions of the district, but that care should 

 also be taken not to introduce species by mistake, or on imper- 

 fect evidence, which might give a false idea of their geographi- 

 cal range, and vitiate the generalizations founded upon them_ 

 Impressed with this conviction, we have been particularly care- 

 ful in scrutinizing the earlier published Catalogues, and com- 

 paring them with the geographical limits of species as far as already 

 known ; in doing this, we have been led to reject some species 

 only found in a dead state, which deference to previous authori- 

 ties might otherwise have induced us to retain. 



One source of error, Avhich, without great care, is likely to vi- 

 tiate our Fauna, is the quantity of shells brought in ballast by 

 the coal ships. This ballast, dredged in the rivers and harbours 

 of the south of England, is, in consequence of the dues payable 

 for its deposit on shore, frequently cast overboard at sea, and 

 shells from it are occasionally washed up on the adjoining sands. 

 We have endeavoured to ascertain as far as possible what shells 

 are attributable to this source, but we cannot be sure that in every 

 case we have succeeded. 



The exposed nature of the coast is not favourable to the use of 

 the dredge, and consequently very few explorations have been 

 made with that useful aid to the naturalist. Our own exploits 

 in that way, we must confess to have been nearly complete failures, 

 owing principally to our inability to stand the exposure and mo- 

 tion of an open sea. Mr. King and Mr. Howse have made at- 

 tempts at deep water dredging, on the cod-fishing grounds at a 

 considerable distance from the shore, the results of which have 

 appeared in the "Annals of Natural History." Though these can 

 scarcely be called successful, there cannot be a doubt that under 

 more favourable circumstances excellent results might be ob- 

 tained from deep-water dredging off our coast, but the expense 

 and discomfort of the undertaking, as prosecuted in the fishing 



